Dell Dimension 8400 XP SP3 problems connecting to modem

Hello. I would need some help on the following problem: my Dell Dimension 8400 XP sp3 does not connect automatically to my ADSL router/modem (Linksys WAG54G9). It connects sometimes at a later stage after I swithced off and on the modem for about 20x. It started after installing sp3. Also a tab 'Authentification' was created but I don't know why. Can anyone help me? I would be very thankful.